1. Field of the Invention
This invention relates to a projector for projecting a video image onto a screen using a plurality of image forming apparatuses such as a cathode ray tube (hereinafter referred to as a CRT) and a liquid crystal display.
2. Description of Related Art
A conventional projector of this type is known the one shown in FIG. 1. This projector is of the so-called 3CRT-1 lens system. In this figure, indicated at 1 (1A, 1B, 1C) are three CRTs. The respective CRTs 1A, 1B, 1C are fabricated for blue, red, and green colors, and all the colors are produced by these three CRTs. Indicated at 2 are dichroic mirrors for reflecting light beams from the CRTs 1, at 3, which is a lens system arranged on an optical path of the reflected light beams from the dichroic mirrors 2, and at 4 a screen. In the front type projectors, the CRTs 1, dichroic mirrors 2, and lens system 3 are constructed integrally. On the other hand, in the rear type projectors, all the elements including the screen 4 are constructed integrally.
There will be next described an operation of thus constructed projector. The respective color light beams from the three CRTs 1A, 1B, 1C are reflected by the two dichroic mirrors 2 utilizing a wavelength selecting property thereof, are incident upon the lens system 3 along the same axis as an optical axis, and are projected onto the screen 4 from the lens system 3.
A projector shown in FIG. 2 is of the so-called 3CRT-3 lens system. In this projector, three lens systems 3 (3A, 3B, 3C) are arranged in correspondence with the CRTs 1A, 1B, 1C.
A spatial relationship between the CRTs 1A, 1B, 1C and the screen 4 of the projector shown in FIG. 1 is shown diagrammatically in FIG. 3. If it is assumed that a direction extending from the CRTs 1 to the screen 4 is a Z-axis, an X-axis exists coplanarly with the Z-axis, and a Y-axis exist on a plane perpendicular to an X-Z plane, tube axes of the CRTs 1A, 1B, 1C (center axes of necks) exist on the X-Z plane and the screen 4 is arranged along the X-Y plane (in reality, the screen 4 may be inclined slightly relative to the X-Y plane).
In the projector of the 3CRT-3 lens system shown in FIG. 2, the CRTs 1A, 1B, 1C are arranged on the X-Z plane and the screen 4 is arranged along the X-Y plane.
The conventional projectors are constructed as described above. Since the CRTs 1A, 1B, 1C are arranged on the X-Z plane perpendicular to the plane of the screen 4, a depth D (D1) of the projector in the direction extending toward the screen 4 increases. Particularly, since the axial length of the CRTs themselves is relatively large, an increase in the depth D (D1) becomes prominent, thereby standing as a hindrance to a smaller size projector.
In view of the above problems, there has been developed a projector as an improvement of the 3CRT-1 lens system in which the tube axis of the CRT 1C is arranged along the Y-axis and the light beam therefrom is reflected by a mirror 5 as shown in FIG. 4. However, this case requires such a special optical path correcting lens 6 as to correct a difference between the optical paths of the CRTs 1A and 1B relative to the CRT 1C, making the construction of the projector complicated.